The isolation, purification and properties of somatotropins are well known in the art. Generally, somatotropin, sometimes referred to as growth hormone in the art, is produced by the pituitary throughout an animal's life. Somatotropin is known to promote skeletal growth, nitrogen retention, protein synthesis and to affect glucose and lipid metabolism. Accordingly, somatotropin is recognized as a general anabolic agent.
Somatotropin can be isolated from excised pituitary tissue. See, e.g., Li, J. Biol. Chem. 211, 55 (1954). Somatotropin can also be obtained from genetically engineered microorganisms containing recombinant DNA which specifies the production of somatotropin. See, e.g., Seeburg, et al., Nature, 276, 795-798 (1978); Seeburg et al., Nature, 270, 486-494 (1978); Martial, Science, 205, 602-607 (1979); and Seeburg, et al., DNA, 2, 37-45 (1983).
Somatotropins from particular species have been studied and characterized. For example, bovine somatotropin is known to be a polypeptide synthesized in and secreted from the anterior lobe of the pituitary. A nucleotide coding sequence and an amino acid sequence of native bovine somatotropin have been reported; e.g. Miller et al., J. Biol. Chem., 255, 7521-24 (1980); and Wallis, FEBS Lett, 35, 11-14 (1973). Bovine somatotropin is a protein of 191 amino acids and appears to be synthesized initially as a bovine pre-somatotropin of 217 amino acids; the signal sequence of 26 amino acids being removed from the N-terminal position during synthesis and secretion, e.g. Lingapa et al., Proc. Natl. Acad. Sci. USA, 74, 2432-36 (1977).
The preparation of bovine somatotropin is well known in the art. For example, bovine somatotropin is extracted from pituitary glands of cattle or produced via recombinant DNA technology in appropriate hosts, e.g., Miller et al., J. Biol. Chem., 255, 7521-24 (1980). U.S. Pat. No. 4,443,539 to Frazier et al, discloses a process for preparing bovine somatotropin by utilizing recombinant DNA methodology to place the bovine somatotropin structural gene into yeast cells. U.S. Pat. No. 4,371,462 to Hecht, discloses a method for the purification of anterior pituitary peptides. European Patent Application Nos. 83304574.3, filed Aug. 8, 1983, with Publication Number 103,395; 82304880.6, filed Sept. 16, 1982, with Publication Number 075,444; and 81303824.7, filed Aug. 21, 1981, with Publication Number 047,600; and British Patent Application No. 2,073,245A disclose methods for producing recombinant bovine somatotropin in high yields. Strains of E. Coli that produce bovine somatotropin are available from the American Type Culture Collection under accession numbers ATCC 31826, 31840, 31841, 31842, and 31843.
Similarly, the preparation of natural and recombinant porcine and human somatotropin is well known. For example, in addition to the publications above which disclose methods for obtaining the porcine and human somatotropin, U.S. Pat. No. 4,604,359 discloses methods for the microbial expression of human somatotropin; U.S. Pat. No. 4,332,717 discloses methods for the purification of human somatotropin; and European Patent Application No. 83305717.7, filed Sept. 26, 1983, with Publication Number 104,920, discloses methods for producing recombinant porcine somatotropin in high yields. U.S. Pat. No. 4,604,359 discloses methods for synthesizing bioactive human somatotropin; including methods for synthesizing a bioactive tetra-S-carbamidomethyl derivative. Many other such publications and methods for various somatotropins are well known to skilled artisans, e.g. U.S. Pat. No. 4,645,755 discloses method for producing fish somatotropin.
Although methods for producing somatotropins are well known, methods for storing somatotropin during the often long period between somatotropin production and use are not well developed. Somatotropins tend to form bioinactive dimers, oligomers, and insoluble aggregates during storage. These bioinactive forms of the somatotropin lower the amount of somatotropin available for use and cause problems during administration, particularly when insoluble aggregates form precipitates in somatotropin solutions.
Methods are, therefore, needed for producing a stable and bioactive somatotropin which will not form bioinactive dimers, oligomers, and aggregates during storage.